Voltage comparison circuits for motion amplitude regulators or the like

ABSTRACT

There is disclosed a driving and control circuit for a tuning fork resonator also adaptable to rotary motor control wherein an approximately sine wave signal from a motion sensor is applied to an instantaneous level detector which causes a control signal to be generated during that portion of the sine wave signal where the instantaneous level exceeds an adjustable reference level; the control signal causes the application of a current to the tuning fork drive which is in a direction to cause braking and reduction of motion amplitude. A distinctive feature of the instantaneous level detector is the fact that the sine wave signal is not rectified but is rather converted to a fluctuating direct current signal with a DC level accurately determined by an internal semiconductor voltage reference element.

This application is a division, of application Ser. No. 016,160, filedMar. 1, 1979.

This invention relates to energy or power control-motion amplituderegulating driving circuits for resonant mechanisms or other apparatus.In the specific example a circuit for driving a tuning fork opticalchopper or scanner is described. The principal distinctive feature ofthe apparatus is a provision for regulation of the amplitude (andvelocity) of the resonant mechanism of the optical chopper.

The electrical apparatus of the present invention is especially welladapted to provide amplitude (or velocity) regulated drive power to aresonant mechanism having a substantial inertial property or "Q", suchas a tuning fork. Quality factor "Q" is defined as two pi times theratio of the maximum stored energy to the energy dissipated per cycle ata given frequency. With mechanical resonators having a "Q" in excess ofabout 100 energy input is effectively integrated regardless of waveform.The effect of a non-sinusoidal wave thus can be equated with a sinewave. A sine wave with an rms drive current approximately equal to 0.9of the peak value of a square wave is its effective equivalent.Similarily, more complex drive waveforms can be considered equivalent toa sine wave of determinable amplitude.

The application of the circuit of the invention to drive mechanicalresonators is the most common anticipated application. The circuitincorporating the invention may also be applied to controlling the driveof rotating machinery, in which case the regulated parameter would bethe velocity of rotation.

In most optical modulating, chopping or scanning applications it isdesirable to have reasonably constant frequency and reasonably constantmotion amplitude. With a properly made self-resonant chopper or scanner,reasonably constant frequency can be achieved by simply stabilizing thestiffness and mass-determining portions of the design which determineself-resonant frequency.

Achieving constant motion amplitude however is not as simple. Basically,chopper or scanner motion amplitude is proportional to device "Q" and(in the case of electromagnetically driven design) also proportional todrive current applied. "Q" is essentially the inverse of energy lossesand the losses can vary depending upon metallurgical friction of bendingmembers, the amount of "out of balance" energy coupled to the base mountmass of the device, the effects of drive and pickup, eddy current andhysteresis losses, and, most significantly, the "windage" losses at highmotion amplitudes. It is not unusual for example for the "Q" of achopper to double and thus motion amplitude double when normalatmosphere windage losses are eliminated by placing it in a vacuum(simulating outer space satellite applications).

Drive current can unintentionally vary depending upon circuit design andcomponent changes and DC input supply voltage changes. A reduction of DCinput supply voltage of 20 percent for example (as might be caused bynear exhausted batteries in a satellite application) could cause a 20percent decrease in drive current and a consequent loss of 20 percent ofdesired motion amplitude.

In essence, the motion amplitude regulation objectives of the disclosedinvention are accomplished by initially applying substantially moredrive current than is actually needed for desired motion amplitude, andthen instantaneously regulating "down" by applying during each cycle ofmotion a reverse "bucking" or "braking current". The braking current ispreferably considerably in excess of original peak drive current and hasa time duration determined by the excess of "attempted" amplitude ascompared to that of desired amplitude. Regulation is done automaticallyas determined by a suitably amplified and phased chopper or scannerpickup voltage (directly proportional to motion amplitude) being appliedto a novel peak level-detecting zener-referenced AC voltage comparatorcircuit. An excess of instantaneous pickup voltage over an adjustablereference level causes braking current to switch on for the period oftime the sinusoidal pickup voltage waveform exceeds the reference level.Contrary to systems where amplitude is averaged over several cyclesbefore being compared with a reference, the present inventioninstantaneously applies a corrective braking current when excess peakamplitude is sensed.

In addition to providing the features and advantages described above itis an object of the present invention to provide an electronic motionamplitude regulator for resonant mechanical devices such as tuningforks, optical choppers, scanners or the like in which excess motionamplitude is sensed in each cycle and a short pulse of braking currentwhich is a function of the excess motion amplitude is applied to thedrive circuit for the resonant mechanical device.

It is another object of the present invention to provide a velocity oramplitude regulator for a mechanical device in which the velocity of thedevice is sensed and excess velocity is caused to produce a brakingcurrent in the drive circuit for the device.

A further object of the present invention is to provide in a motionamplitude regulator a capacitively input-coupled zener-referenced peakAC voltage level detector having a symmetrical input impedance whichpreserves the integrity of the zener determined DC level.

Other objects and advantages of the present invention will be apparentfrom consideration of the following description in conjunction with theappended drawings in which:

FIG. 1 is a schematic circuit diagram of motion amplitude regulatorapparatus according to the invention;

FIGS. 2A, 2B and 2C are waveform diagrams of motion amplitude, drivecurrent, and pickup voltage useful in explaining the operation of theapparatus of FIG. 1.

Referring to FIG. 1 an optical chopper 11 is shown schematically whichconsists essentially of a tuning fork having a pair of vanes or shuttersmounted thereon between which a light beam will be passed. Theparticular construction of the optical chopper 11 is not relevant to thepresent invention and in fact the motion amplitude regulator of theinvention may be applied to control of numerous forms of resonant ornon-resonant devices which are electromagnetically driven.

A drive coil 13 is provided for the tuning fork of chopper 11; coil 13has terminals 15 and 17. Terminal 15 is connected by lead 19 to a DCpower supply which may, for example, be a 15 volt regulated powersupply.

Terminal 17 of drive coil 13 is connected through a resistor 21 to thecollector of a transistor 23. The emitter of transistor 23 is connectedthrough resistor 25 to ground. Thus, when transistor 23 is conducting asa result of the signal applied to its base, drive current will flowthrough drive coil 13, resistor 21, transistor 23 and resistor 25 toground.

The drive circuit for the chopper is a conventional self-biasingcircuit. A pickup coil 33 having terminals 35 and 37 provides the pickupsignal for the regenerative oscillator circuit. Terminal 35 of pickupcoil 33 is connected through a filter circuit consisting of resistors37, 39 and capacitors 41, 43, to the positive end of resistor 25. Uponinitial application of plus DC voltage resistor 25 biases the collectorof transistor 23 midway between on and off thus permitting initialmaximum loop gain for starting oscillation.

The voltage of pickup coil 33 appears at the ungrounded terminal ofcapacitor 45 and is amplified in the proper phase relation inconventional operational amplifier 47. Amplifier 47 may have a gain ofapproximately 40 which is more than sufficient to apply an amplifiedinverted pickup signal to the base of transistor 23 causing transistor23 to supply a substantially square wave pulse drive current of properphase to drive coil 13.

The apparatus thus far described is a generally conventional drivecircuit for a tuning fork oscillator or optical scanner. As previouslyexplained generally the conventional drive circuit may produce aconstant frequency oscillation due to the inherent frequency stabilityof the tuning fork or optical scanner but amplitude of the resonantmechanical device is not controlled or regulated by the conventionaldrive circuit.

In order to regulate the amplitude of the resonant mechanical device thecircuit of FIG. 1 takes an output signal from pickup coil 33 over lead51 through capacitor 53 to amplifier 55. Amplifier 55 is a conventionaloperational amplifier with a feedback resistor 57 connected from itsoutput to its input. The feedback input of amplifier 55 is connectedthrough resistor 59, variable resistor 61 and capacitor 63 to ground.Adjustment of resistor 61 to increase its resistance decreases the gainof amplifier 55 and as will later be seen produces an increase in theregulated motion amplitude. Amplifier 55 may have a nominal gain ofabout 23.

As previously explained it is the purpose of the motion amplituderegulation circuit to detect motion amplitude in excess of the desiredmotion amplitude and apply a braking current to drive coil 13 foroptical chopper 11. The output of amplifier 55 is a linear function ofthe pickup coil voltage which is essentially proportional to the tinevelocity of the optical chopper 11. Since peak motion amplitude of theoptical chopper 11 is proportional to the peak tine velocity the outputof amplifier 55 has a peak which is essentially a linear function of theoptical chopper motion amplitude. Since the output of amplifier 55 isactually responsive to velocity rather than amplitude it is 90 degreesout of phase with the instantaneous amplitude of the optical choppertine motion; this, however, is not material to the operation of thecircuit because the important phase relationship is that between theoutput signal of pickup coil 33 (and of amplifier 55) relative to thedrive current supplied to drive coil 13.

From the previous discussion it will be seen that it is desired tocompare the positive going instantaneous level of the output ofamplifier 55 with a reference voltage. The reference voltage is suppliedby a zener diode 65 which may have a reference voltage of 6.4 volts forexample. Resistors 67 and 69 form a voltage divider which produces afraction of the reference voltage of zener diode 65 (for example 4.5volts) at the base of a transistor 71 to which is also connected theoutput of amplifier 55 through a capacitor 73. Transistor 71 is a PNPtransistor with its collector grounded and its emitter connected throughresistor 75 to the positive DC power supply.

The emitter of transistor 71 is connected to the base of an NPNtransistor 77 having its emitter connected to the positive terminal ofzener diode 65. The positive terminal of zener diode 65 is connectedthrough a resistor 79 to the positive power supply. The collector oftransistor 77 is connected through a resistor 81 to the base of atransistor 83 which is also connected through a resistor 85 to thepositive power supply. Transistor 83 is a PNP transistor. The emitter oftransistor 83 is connected through a resistor 87 to the positive powersupply while its collector is connected through a resistor 89 to thebase of a switching transistor 91.

The base of transistor 91 is connected through a resistor 93 to ground;the emitter of transistor 91 is connected to ground. The collector oftransistor 91 is connected through a resistor 97 to the terminal 17 ofdrive coil 13 to provide an alternate current path for drive currentthrough coil 13, resistor 97, and transistor 91 to ground.

The zener diode 65 in conjunction with voltage dividing resistors 67 and69 set a DC level for the output of amplifier 55 at 4.5 volts at thebase of transistor 71. At the same time a level of 6.4 volts is set forthe emitter of transistor 77 which is paired with transistor 71.Therefore, when the AC output of amplifier 55 coupled through capacitor73 slightly exceeds a positive instantaneous value of 1.9 volts (6.4volts minus 4.5 volts), transistor 71 starts to turn off turning ontransistor 77 and in turn transistor 83 and finally switching transistor91 causing "reverse" braking current to flow through drive coil 13.While the level detecting circuit may be considered to be a peakdetector, it might be more accurately designated an instantaneous leveldetector because it deactivates on the down-going side of the peak.

It is important to note that there is no DC level shift due to thechange in the level of the output of amplifier 55 nor is there any DClevel shift from changes in power supply voltage. Thus, the peak(positive) value of the AC output from amplifier 55 (which is apredetermined multiple of the pickup coil output) is accuratelydetermined. The positive peak of slightly more than 1.9 voltscorresponds to about 1.4 volts rms. With a gain for amplifier 55 ofabout 23, the regulated level of output of pickup coil 33 is about 61millivolts rms.

The remarkable efficacy of the AC instantaneous level comparator circuitof the invention can only be appreciated in terms of the problemsexisting with previous circuits. The basic objective of the circuit isto measure the AC peak level and consequently one utilizes capacitivecoupling as represented by capacitor 73 to eliminate any effect from thelevel of the DC component at the output of amplifier 55. That techniqueis also used in prior circuits.

However, prior circuits commonly have next rectified the AC signalcoupled through the coupling capacitor. As a result the input impedanceviewed looking forward from the coupling capacitor is dependent upon theinstantaneous voltage level. There is a certain threshhold below whichcurrent is not drawn by the input to the next stage whereas above thatlevel significant current is drawn. Such an asymmetrical current draincharges the coupling capacitor at the input and produces an errorbecause DC level is thereby affected by the magnitude of the AC signal.It has been suggested that this problem can be remedied by inserting abuffer stage in the circuit but such remedy may itself create otherproblems.

In the circuit of the invention a different approach is used whichessentially avoids rectification of the AC signal by causing themid-value of the AC voltage waveform to be raised to a specific,accurately regulated value.

As seen in FIG. 1 the zener diode designated 65 produces an accuratelycontrolled voltage (in this case 6.4 volts) at the emitter of transistor77. The resistors 67 and 69 form a voltage divider between the emitterand ground, the center tap of which is connected to the input at thebase of transistor 71. The voltage divider center tap voltage thusdetermined (in this case 4.5 volts) causes the DC level seen bytransistor 71 to be accurately set at a fixed value which does not varywith the AC level.

The turn on of the braking current by switching transistor 91 will bebetter understood by reference to FIGS. 2A, 2B, and 2C. In FIG. 2A theapproximately square waveform A represents the drive current to coil 13without motion amplitude regulation; this situation would prevail duringstartup of optical scanner oscillations before buildup of the desiredmotion amplitude. For comparison purposes the chopper amplitude waveformis shown at B in FIG. 2A. It will be noted that the amplitude waveformrepresenting physical displacement of the chopper vanes is (lagging) 90degrees out of phase with the drive current.

The convention used in 2A is that the upper level of waveform Arepresents drive current with transistor 23 off while the bottom levelrepresents drive current with transistor 23 on.

Waveform C in FIG. 2A represents the output of pickup coil 33 which isessentially a linear function of velocity. Waveform C leads waveform Bby 90 degrees and is thus in phase with waveform A. The waveform frompickup coil 13 could, of course, be reversed (shifted) 180 degrees by areversal of the connections of terminals 35 and 37.

FIGS. 2B and 2C show the operation of the braking current switchingcontrolling the optical scanner motion amplitude. It will be noted inFIG. 2B that the instantaneous amplitude of the pickup waveform C hasvery slightly exceeded the level D determined by the gain of amplifier55 and the detection level of the instantaneous level detector circuit.Thus, for approximately 10 degrees before and after the peak of waveformC to instantaneous level detector circuit will cause transistor 91 to beturned on. This occurs during the period that drive transistor 23 isoff. However, the turn on of transistor 91 causes a pulse of drivecurrent indicated at E for a duration of about 20 degrees. The timing ofpulse E makes it 180 degrees out of phase with the normal drive currentthus producing a braking effect diminishing the motion amplitude of theoptical chopper. The greater the width of pulse E the greater thebraking effect. Thus, as shown in FIG. 2C, when the amplitude of thepickup waveform C is still more in excess of the level D, the brakingpulse F which is correspondingly wider has a still greater brakingeffect.

It will be noted that the braking pulses E and F are of greateramplitude than the normal drive pulse of waveform A. This is due to thefact that resistor 97 has a value substantially less than resistor 21.For example, the relative values of resistors 21 and 97 may bedetermined so that the combination of the resistance of resistor 97 andthe drive circuit resistance is about 50 percent of the combination ofthe resistance of 21 and the drive circuit resistance. This factor canbe further reduced to about 20 percent beyond which there appears to belittle effect. The effect of greater magnitude of braking pulses E and Fis that much greater braking effect may be achieved from the same width(or narrower) braking pulse. The energy per unit time (or the power)provided by drive current or braking current is proportion to the squareof the current amplitude. Therefore, if braking current is twice drivecurrent its effect is four times as great over the same time period.Hence the braking time period can be shortened to produce a givenbraking power required to regulate the amplitude. If adequate braking isachieved by a narrow pulse the regulation is much more accurate and willbe maintained within a few percent or less.

It is worthy of note that the operation of the instantaneous detectioncircuit is such that only a very small excess motion amplitude producesa relatively wide braking pulse and thus the range of regulation issmall and the regulation accuracy good. For example, a motion amplitudeexcess of one percent will cause the pickup waveform to intercept thedetection level at about eight degrees before and after the peak orcenter of the waveform. Thus, the braking current pulse may last almost10 percent of one-half cycle of the driving current when there is only aone percent excess motion amplitude relative to the setting of the peaklevel detector. Ten percent pulse time will produce a much greater than10 percent braking power due to the larger braking current and thesquare of current effect previously discussed. In practice, of course,the value for resistor 21 and other circuit parameters will be set sothat there is always some excess drive current and always a brakingpulse of greater or less width as required to bring the motion amplitudeback to the desired level. Depending on the particular application theexcess drive current produced in the absence of braking pulses producedby transistor 91 may be from about 10 percent to several hundred percentor more. In determining the excess drive current provided by the drivecircuit and transistor 23 one requires consideration of the type offactors which may be expected to bring the regulator into operation. Forexample, a drop in power supply voltage due to partially dischargedbatteries as might occur in a satellite application would call forupside regulation of the motion amplitude. On the other hand, reductionof air pressure and reduction of wind drag on the optical chopperproduces a tendency to increase motion amplitude and requires downsideregulation. Downside regulation does not require an excess of drivecurrent whereas upside regulation does require an initial excess ofdrive current.

The motion amplitude regulator circuit of the invention will be seen tobe particularly adapted to control tuning forks, optical choppers,scanners and similar resonant apparatus wherein the "Q" of themechanically resonant apparatus is relatively high, say over 100.Usually, the "Q" of such apparatus will be a thousand or more. The high"Q" means that the energy stored in the oscillating mechanical resonatoris high compared to the energy which must be provided to maintain asteady oscillation. Therefore, the waveform of the cyclic pulses whichdrive the resonator is of little importance since a single pulseproduces little change in the energy of the system and consequentlylittle change in the motion amplitude or the velocity. At the same timeit is very beneficial to detect small changes in the instantaneousvelocity (which is proportional to amplitude) and apply immediatecorrection rather than to produce an average value requiring integrationfor some time before a suitable correction could be initiated.

It has been found for example that when the motion amplitude regulationcircuit of the invention is applied to a mechanically resonant devicesuch as an optical chopper extremely accurate motion amplituderegulation can be achieved. Such a chopper which experiences a 100percent increase in amplitude upon evacuation of the chopper housingwith the consequent decrease in air drag is found to be regulated toonly a three percent increase in motion amplitude when the circuit ofthe invention is incorporated in the drive for the chopper. Similarilysuch a chopper which experiences plus or minus 20 percent motionamplitude variation with DC input voltage changes is limited to a motionamplitude variation of plus or minus two percent with the circuit of theinvention.

Given the foregoing description and explanation it is beleived thatdesign of circuits according to the invention for specific applicationswould be a straightforward exercise for one skilled in the art; as afurther aid in understanding the invention the Table I below showsexemplary circuit element values and semi-conductor type numbers.

                  TABLE I                                                         ______________________________________                                        ELEMENT                                                                       REF. NO.            VALUE                                                     ______________________________________                                        RESISTANCE:         OHMS:                                                     13                  200                                                       21                  1200                                                      25                  71.5                                                      37                  100.                                                      39                  1000.                                                     57                  20K                                                       59                  470                                                       61                  2000                                                      67                  4220                                                      69                  10K                                                       75                  47K                                                       79                  2700                                                      81                  4300                                                      85                  2400                                                      87                  200                                                       89                  6200                                                      93                  4700                                                      97                  536                                                       CAPACITANCE:        MICROFARADS:                                              41,43               56.                                                       45                  .01                                                       53                  6.8                                                       63                  47.                                                       SEMICONDUCTORS:     TYPE NUMBERS:                                             23                  2N2219A                                                   71,83               2N2907A                                                   77,91               2N2222A                                                   65                  1N4576A                                                   ______________________________________                                    

It will be understood by those skilled in the art that the presentinvention also may be applied to rotary electric motors. Anelectromagnetically driven tuning fork resonator is, of course, anelectric motor also, but it produces vibratory motion rather than rotarymotion. The conceived applicability of the instant motion amplituderegulation circuitry to rotary motors would be to DC motors rather thansynchronous or other alternating current motors. For example aconventional DC motor with a split-ring commutator may be provided witha shaft mounted magnetic gear tooth arrangement together with a magneticpickup coil to produce a velocity sensitive sine wave in response tomotor shaft motion (this is in effect an AC tachometer). This sensorwould be analagous to the tuning fork pickup coil. Excess velocity ofthe motor would cause the level comparator to trigger the motor's powersupply to produce a negative or braking current which should be on theorder of twice the motor forward drive current. In the case of arotating motor it is possible to obtain large braking currents by meansof a near short circuit of the motor armature windings due to the largereverse current produced by the motor back emf. This is the well knowndynamic braking effect. The dynamic braking effect could be utilizedunder control of the instantaneous AC level comparator circuiteliminating any necessity for applying a reverse voltage.

Furthermore, the invention may be particularly applicable to electricmotors employing electronic commutation. In such case an electromagneticor other sensor of the shaft position and velocity for controllingcommutation may also be employed to provide the AC (sine wave) voltagesignal supplied to the instantaneous AC level comparator circuit.

From the foregoing explanation it will be seen that according to thepresent invention a circuit is provided which serves to regulate themotion amplitude of a resonant mechanical device in a particularlyefficient and effective manner. It does not require any additionalmotion sensor or pickup other than the conventional pickup coil(although a separate sensor could be utilized if that was everdesirable). Also, the braking current which controls the motionamplitude is conducted through the conventional drive coil in thepreferred embodiment without requiring addition to or alteration of thedrive mechanism for the resonant mechanism. Again, should there be anyreason for it a separate drive coil could be provided for the brakingcurrent. The amplifier circuits and switching circuits employed in theapparatus may in many cases be replaced by equivalent circuits. Thereis, however, a distinct advantage in the AC instantaneous level detectorcircuit which avoids the effect of DC level shift so that the AC signalrepresenting the motion amplitude is accurately measured and comparedwith the internal zener diode reference.

In addition to the variations and modifications to the preferredembodiment of the apparatus shown or suggested herein other variationsand modifications will be apparent to those skilled in the art andaccordingly the scope of the invention is not to be deemed to be limitedto those variations and modifications of the invention shown orsuggested but is rather to be determined by reference to the appendedclaims.

What is claimed is:
 1. A voltage comparison circuit comprising,anamplifier producing a fluctuating voltage DC signal, means for isolatingthe AC component of said signal, a semiconductor switch circuit havingan input connected to receive said AC component of said signal, saidswitch circuit being adapted to operate from a fixed voltage DC powersupply, means for establishing a DC level of said AC component includinga voltage reference element with a reference voltage substantially lessthan the voltage of said power supply and a voltage dividing circuit inparallel therewith having its divided voltage point connected to saidinput of said semiconductor switch circuit, and means for connecting theundivided voltage of said voltage reference element to bias saidsemiconductor switch circuit, whereby the output of said semiconductorswitch circuit provides an instantaneous AC voltage level-detecting,internally-referenced voltage comparison circuit.
 2. Apparatus asrecited in claim 1 wherein said voltage reference element comprises,azener diode.
 3. Apparatus as recited in claim 1 wherein said switchcircuit comprises,a transistor with its base being the switch circuitinput and said divided voltage point is connected to the base of saidtransistor by a conductive element.
 4. Apparatus as recited in claim 1wherein said switch circuit is powered from a source not voltageregulated from said voltage reference element.
 5. Apparatus as recitedin claim 1 wherein said amplifier is an adjustable gain amplifier.
 6. Avoltage comparison circuit to be operated from a DC power supplycomprising,an amplifier producing a fluctuating voltage DC signal, meansfor selectively transmitting the AC component of said signal, asemiconductor switch circuit having an input connected to receive onlysaid AC component of said signal, means for establishing a DC level ofsaid AC component including at least one voltage reference element witha reference voltage substantially less than the voltage of said powersupply and a voltage dividing circuit in parallel with one of said atleast one voltage reference element having its divided voltage pointconnected to said input, and means for connecting the voltage of one ofsaid at least one voltage reference element to bias and semiconductorswitch circuit.
 7. Apparatus as recited in claim 6 wherein one of saidvoltage reference elements comprises,a zener diode.
 8. Apparatus asrecited in claim 6 wherein said switch circuit comprises,a transistorwith its base being the switch circuit input and said divided voltagepoint is connected to the base of said transistor by a conductiveelement.
 9. Apparatus as recited in claim 6 wherein said amplifier is anadjustable gain amplifier.
 10. A voltage comparison circuit to beoperated from a DC power supply comprising,an amplifier producing afluctuating voltage DC signal, means for selectively transmitting the ACcomponent of said signal, a semiconductor switch circuit with an inputconnected to receive said AC component without rectification, means forestablishing a non-zero DC level of said AC component including avoltage reference element with a reference voltage substantially lessthan the voltage of said power supply and a voltage dividing circuitcomprising two resistors connected in series with each other and inparallel with said reference element, the junction of said resistorsbeing connected to establish the DC level at said input of saidsemiconductor switch circuit, and means for connecting the voltage of avoltage reference to determine a detection level for said semiconductorswitch circuit, whereby the output of said semiconductor switch providesan instantaneous AC voltage level-detecting, internally-referencedvoltage comparison circuit.
 11. A voltage comparison circuitcomprising,an amplifier producing a fluctuating voltage DC signal, meansfor selectively transmitting the AC component of said signal, asemiconductor switch circuit with an input connected to receive said ACcomponent without rectification, a source of DC voltage to power saidswitch circuit, means for establishing a non-zero DC level of said ACcomponent including a voltage reference element with a reference voltagesubstantially less than the voltage of said source and a voltagedividing circuit in parallel therewith having its divided voltage pointconnected to establish the DC level of said input of said semiconductorswitch circuit, and means for connecting the voltage of a voltagereference to determine a detection level for said semiconductor switchcircuit, whereby the output of said semiconductor switch circuitprovides an instantaneous AC voltage level-detecting,internally-referenced voltage comparison circuit.
 12. Apparatus asrecited in claim 11 wherein said voltage reference element comprises,azener diode.
 13. Apparatus as recited in claim 11 wherein said switchcircuit comprises,a transistor with its base being the switch circuitinput and said divided voltage point is connected to the base of saidtransistor by a conductive element.
 14. Apparatus as recited in claim 11wherein said amplifier is an adjustable gain amplifier.